Method for making a strain-relieved tunable dielectric thin film

ABSTRACT

Tunable dielectric thin films are provided which possess low dielectric losses at microwave frequencies relative to conventional dielectric thin films. The thin films include a low dielectric loss substrate, a buffer layer, and a crystalline dielectric film. Barium strontium titanate may be used as the buffer layer and the crystalline dielectric film. The buffer layer provides strain relief during annealing operations.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a divisional application of U.S. patentapplication Ser. No. 09/834,327, filed Apr. 13, 2001, now pending.

FIELD OF THE INVENTION

[0002] The present invention relates to tunable dielectric thin films,and more particularly relates to strain-relieved and defect-reducedtunable dielectric thin films which significantly reduce dielectric lossat microwave frequencies.

BACKGROUND INFORMATION

[0003] Considerations in the development of tunable microwave devicesbased on ferroelectric materials are the dielectric constant, tunabilityand the dielectric quality factor (Q=1/tan δ) of the materials. The DCelectric field dependent dielectric constant of ferroelectric thinfilms, such as Ba_(1-x)Sr_(x)TiO₃ (BST, 0≦x≦1), is currently being usedto develop low loss tunable microwave devices, such asvoltage-controlled oscillators, tunable filters and phase shifters. Thisresults from disadvantages associated with currently available tunablemicrowave devices based on PIN diodes and ferrites. Currentsemiconductor-based devices exhibit substantial losses at frequenciesover 2 GHz, and high power is needed to operate current ferrite-baseddevices.

[0004] The provision of low loss tunable microwave devices based onferroelectric thin films would reduce the size and operating power ofdevices while providing wide bandwidth and narrow beamwidth. One of themost critical properties that should be maximized in these applicationsis the dielectric quality factor of the ferroelectric thin films whilemaintaining a reasonable change in the dielectric constant with low DCelectric fields at high frequencies (i.e., ≧2 GHz).

[0005] Although attempts have been made at developing tunable dielectricthin films having low dielectric losses, and improvements have been madeat low frequencies (≦1 MHz), prior art methods are not conducive to theuse of these films in tunable applications at high frequencies (i.e., ≧2GHz). Tunable dielectric materials having significantly increased Qvalues at low frequencies (≦1 MHz) developed in the prior art are notcommercially viable because currently available semiconductor materialshave much better performance at those frequencies than theconventionally developed dielectric materials. A dielectric thin filmferroelectric device with optimal characteristics for tunable microwaveapplications has not yet been provided in the prior art.

SUMMARY OF THE INVENTION

[0006] The present invention provides dielectric thin films forapplications such as electronically tunable devices having tuningspecified for each device (i.e., voltage-controlled oscillators, tunablefilters, phase shifters, etc.) and a high quality factor at highfrequencies (≧2 GHz). A strain-relieved tunable dielectric thin film isprovided which minimizes a strain-enhanced inverse relationship betweendielectric tuning and dielectric Q. The present invention providesstrain-relieved and defect-reduced dielectric films that exhibitdesirable dielectric properties at high frequencies (i.e., ≧2 GHz),which can be used in applications such as tunable microwave devices. Thepresent invention also provides for annealing of the film materialwithout thermally induced unit cell distortion caused by film strain dueto thermal expansion mismatch between the film and substrate.

[0007] A process in accordance with an embodiment of the presentinvention includes the steps of: (i) forming a thin (e.g., <1,000 Å)buffer layer such as BST (i.e., any porous phase between partiallycrystallized amorphous phase and fully crystallized randomly orientedphase) on a crystalline, low dielectric loss substrate by a lowtemperature deposition technique and a subsequent heat treatment; (ii)depositing a second layer (e.g., 5,000 Å) of highly crystallizedrandomly oriented BST film on top of the BST buffer layer at a hightemperature (e.g., 750° C.); and (iii) annealing the film to reducedeposition-related crystalline defects (e.g., oxygen vacancies) and togrow crystalline grains.

[0008] In accordance with an embodiment of the present invention, thethin BST buffer layer relieves the film strain caused by film/substratemismatches, i.e., lattice and thermal expansion mismatches between thefilm and substrate during the film deposition process and post-annealingprocess. The strain-relieved and defect-reduced tunable dielectric thinfilm according to the invention provides higher dielectric qualityfactors at high frequencies (≧2 GHz) than currently existing prior artsemiconductor materials as well as other prior art ferroelectricmaterials. Thus, the present invention provides a tunable dielectricthin film for tunable microwave applications, and a method of formationthereof, which relieves film strain having a significant effect on bothmicrostructure and microwave dielectric properties of the film.

[0009] An aspect of the present invention is to provide a tunabledielectric thin film comprising a low dielectric loss substrate, abuffer layer on the low dielectric loss substrate, and a crystallinedielectric film on the buffer layer.

[0010] Another aspect of the present invention is to provide a method ofmaking a thin film dielectric material. The method comprises the stepsof depositing a thin dielectric buffer layer on a low dielectric losssubstrate at a first temperature, and depositing a layer of dielectricthin film on the dielectric buffer layer at a second temperature. Thedeposited layers may be annealed to reduce crystalline defects and togrow crystalline grains.

[0011] These and other aspects of the present invention will be moreapparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIGS. 1a-1 c schematically illustrate manufacturing process stepsin accordance with an embodiment of the present invention.

[0013]FIGS. 2a-2 d are scanning electron microscope (SEM) images of aBST buffer layer, a BST dielectric film deposited on the BST bufferlayer, and an annealed BST film.

[0014]FIGS. 3a-3 d are x-ray diffraction (XRD) patterns of the BSTbuffer layer of FIGS. 2a and 2 b, respectively, the BST film of FIG. 2d,and the JCPDS card #34-0411 for Ba_(0.6)Sr_(0.4)TiO₃ powder.

[0015]FIGS. 4a and 4 b are x-ray diffraction (XRD) patterns of (a)symmetric (004) peaks and (b) asymmetric (024) peaks of epitaxial BSTfilm deposited on (001) MgO without a BST buffer layer.

[0016]FIGS. 5a and 5 b are partially schematic isometric views oftunable dielectric varactors based on (a) epitaxial BST film depositedon (001) MgO without a BST buffer layer, and (b) randomly orientedpolycrystalline BST film deposited on (001) MgO with a BST buffer layer.

[0017]FIGS. 6a and 6 b are resonance curves of a stripline resonatorincorporating planar varactors of BST films grown (a) without and (b)with a BST buffer layer at different applied DC bias voltages up to 200V at room temperature.

[0018]FIGS. 7a and 7 b are plots of (a) percentage tuning of capacitanceand (b) Q of varactor as a function of varactor gap size at 2 GHz and atDC bias voltages up to 300 V.

[0019]FIGS. 8a and 8 b are plots of (a) percentage tuning of capacitanceand (b) Q of varactor as a function of varactor gap size at 8 GHz and atDC bias voltages up to 300 V.

DETAILED DESCRIPTION

[0020] The present invention provides a tunable dielectric thin filmincluding: a low dielectric loss substrate; a buffer layer on the lowdielectric loss substrate; and a crystalline dielectric film on thebuffer layer. A process to relieve film strain in crystalline BST filmis also provided which utilizes a thin BST buffer layer (i.e., any BSTphase from amorphous to randomly oriented crystalline phase). Suchstrain-relieved films using this method may be heat-treated withoutthermally induced distortion which would otherwise result due to thermalexpansion mismatch between the film and substrate, e.g., duringannealing

[0021]FIGS. 1a-1 c schematically illustrate a manufacturing process of adielectric thin film according to the present invention. In FIG. 1a, abuffer layer 12 is deposited on a low dielectric loss substrate 10. InFIG. 1b, a crystalline dielectric film 14 is deposited on the bufferlayer 12. The buffer layer 12 and crystalline dielectric thin film 14may be deposited by techniques such as RF sputtering, pulsed laserdeposition, metal-organic chemical vapor deposition and the like. InFIG. 1c, the crystalline dielectric film 16 has been annealed. Forexample, the method illustrated in FIGS. 1a-1 c may comprise the stepsof BST buffer layer deposition, strain-relieved crystalline BST filmdeposition on top of BST buffer layer, and strain-relieved orsubstantially strain free and defect-reduced BST film preparationthrough annealing.

[0022] The low dielectric loss substrate 10 may have a thickness of, forexample, from about 250 to about 500 micron or more, with thicknesses ofabout 500 micron being suitable for many applications. The buffer layer12 may typically have a thickness of from about 0.0005 to about 0.2micron, for example, from about 0.01 to about 0.05 or 0.1 micron. Thecrystalline dielectric film 16 may have a typical thickness of fromabout 0.1 to about 5 micron, for example, from about 0.3 to about 1micron.

[0023] As a dielectric film is strained during film deposition processesand post-annealing processes due to thermal expansion mismatch betweenthe film and substrate, conventional films do not have desirabledielectric properties exhibiting both high dielectric Q and highdielectric tuning because of the film strain, even though structuraldefects are significantly reduced and crystalline grains are largelygrown in the film during the post-annealing. For example, U.S. Pat. No.6,096,127 to Dimos et al. entitled “Tunable Dielectric Films having LowElectrical Losses” discloses that reduction in loss is realized bydramatically increasing the grain size of the dielectric films. Theincrease in grain size is realized by heating the film to a temperatureat which the grains experience regrowth. However, after post-annealing,the Q value at 0 V_(DC) of the epitaxial dielectric films grown onLaAlO₃ substrates are even lower (Q_(0V)=30) with higher dielectrictuning (64-69% at E=3-5 V/μm) at 1-2 GHz than the films withoutpost-annealing (i.e., Q_(0V)=40-50 with a 36-38% dielectric tuning).Apparently, the resulting dielectric properties result because, afterpost-annealing, the epitaxial films are strained further into anin-plane tetragonal distortion due to thermal expansion mismatch betweenthe film and substrate, even though film grains are expected to growduring the post-annealing process.

[0024] It has been observed that epitaxially grown BST of the formulaBa_(1-x)Sr_(x)TiO₃ (where BST x=0.3-0.6) films on (100) MgO and LaAlO₃substrates are tetragonally distorted at room temperature, although thestructure for the corresponding bulk is cubic, and the latticedistortion has a significant impact on the microwave dielectricproperties. Both experimental results based on planar capacitorsincorporating epitaxial BST films deposited on (100) MgO and LaAlO₃substrates and theoretical analyses for the experimental configuration(i.e., in-plane properties of the films) based on Devonshirethermodynamic theory show that the structural distortion caused by filmstrain is one of factors enhancing an inverse relationship betweendielectric tuning and dielectric Q at microwave frequencies. The inverserelationship enhanced in epitaxial BST films results in either a largedielectric tuning (>75%) with a low dielectric Q (<50), or a highdielectric Q (>200) with a small dielectric tuning (<10%) with areasonably small DC bias voltage (i.e., 50 V) and gap size (i.e., 5 μm)of a planar structure at microwave frequencies, depending on eitherin-plane tetragonal distortion or normal tetragonal distortion,respectively. Therefore, as long as a substantial strain-inducedstructural distortion exists in the film, the microwave dielectricproperties of the film cannot exhibit both large dielectric tuning andhigh dielectric Q at the same time. Film strain should therefore berelieved to improve the dielectric properties.

[0025] The present invention provides a process for forming tunabledielectric thin films having high dielectric Q for use in tunablemicrowave applications through the formation of strain-relieved anddefect-reduced tunable dielectric thin films. Preferred thin filmdielectric materials include Ba_(1-x)Sr_(x)TiO₃ (where 0≦x≦1) which is asolid-solution ferroelectric material exhibiting a DC electricfield-dependent dielectric constant and a Ba/Sr composition-dependentCurie temperature. Preferred thin film dielectric materials for tunablemicrowave applications at room temperature have Ba/Sr compositions ofx=0.4 (i.e., Ba_(0.6)Sr_(0.4)TiO₃) and x=0.5 (i.e.,Ba_(0.5)Sr_(0.5)TiO₃). The barium strontium titanate materials mayoptionally include other materials, such as dopants and/or other metaloxides.

[0026] Other tunable dielectric materials may be used partially orentirely in place of barium strontium titanate. Examples includeBa_(x)Ca_(1-x)TiO₃ (0.2≦x≦0.8), Pb_(x)Zr_(1-x)SrTiO₃ (0.05≦x≦0.4),KTa_(x)Nb_(1-x)O₃ (0≦x≦1), Pb_(x)Zr_(1-x)TiO₃ (0≦x≦1), and mixtures andcomposites thereof. Also, these tunable dielectric materials can becombined with low loss dielectric materials, such as magnesium oxide(MgO), aluminum oxide (Al₂O₃) and zirconium oxide (ZrO₂) and/or withadditional doping elements, such as manganese (Mn), iron (Fe), andtungsten (W) or other alkali earth metal oxide (i.e., calcium oxide,etc.), transition metal oxides (i.e., manganese oxide, iron oxide,hafnium oxide, tungsten oxide, etc.), rare earth metal oxides,silicates, niobates, tantalates, aluminates, zirconates, and titanatesto further reduce the dielectric loss. The buffer layer may also bebarium titanate, strontium titanate, barium calcium titanate, bariumcalcium zirconium titanate, lead titanate, lead zirconium titanate, leadlanthanum zirconium titanate, lead niobate, lead tantalate, potassiumstrontium niobate, sodium barium niobate/potassium phosphate, potassiumniobate, lithium niobate, lithium tantalate, lanthanum tantalate, bariumcalcium zirconium titanate or sodium nitrate.

[0027] Suitable low dielectric loss substrates include magnesium oxide(MgO), aluminum oxide (Al₂O₃) and lanthanum aluminum oxide (LaAl₂O₃).Suitable buffer layers may be made of the same or similar compositionsas the crystalline dielectric thin films deposited thereon. Examplesinclude Ba_(1-x)Sr_(x)TiO₃ (0≦x≦1), Ba_(x)Ca_(1-x)TiO₃ (0.2≦x≦0.8),Pb_(x)Zr_(1-x)SrTiO₃ (0.05≦x≦0.4), KTa_(x)Nb_(1-x)O₃ (0≦x≦1),Pb_(x)Zr_(1-x)TiO₃ (0≦x≦1), and mixtures and composites thereof.

[0028] Two tests were conducted. Example 1 included tunable dielectricthin film deposition using pulsed laser deposition (PLD), film structurecharacterization using x-ray diffraction (XRD) and scanning electronmicroscopy (SEM), and post-deposition film annealing. Example 2 includedsingle-gap planar electrodes preparation on top of the film using aphotolithography lift off technique and dielectric property measurement(i.e., dielectric tuning and dielectric Q) at 2 and 8 GHz usinggap-coupled microstrip resonators connected to an HP network analyzer.

EXAMPLE 1

[0029] A number of tunable dielectric thin films were fabricated todemonstrate the superior dielectric properties at microwave frequenciesof the film according to the present invention over currently existingprior art semiconductor materials as well as other prior artferroelectric materials. First, a thin (<0.1 μm thick) amorphous BST(x=0.4) buffer layer was deposited on (001) MgO low dielectric loss,single crystal substrate at room temperature in an oxygen backgroundpressure of 200 mTorr by pulsed laser deposition (PLD). The output of ashort-pulsed (30 ns full width at half maximum) eximer laser operatingwith KrF (λ=248 nm) at 1 Hz was focused to a spot size of ˜0.1 cm² andan energy density of 1.9 J/cm² onto a single-phase BST (x=0.4) target.The vaporized material was deposited onto (001) MgO substrateapproximately 5 cm away from the target. The microstructure of BSTbuffer layer can be modified into any phase between an almost amorphousphase and a randomly oriented fully crystallized phase, depending onheat treatment after the buffer layer deposition at room temperature.

[0030]FIG. 2a and FIG. 3a show a SEM image and a XRD pattern,respectively, of a BST (x=0.4) buffer layer deposited at roomtemperature and then heated at 750° C. for 1 min in an oxygen backgroundpressure of 200 mTorr. The microstructure turns out as an almostamorphous phase with an extremely small size of perovskite BST phasenucleation. On the other hand, FIG. 2b and FIG. 3b show a SEM image anda XRD pattern, respectively, of a BST (x=0.4) buffer layer deposited atroom temperature and then annealed at 900° C. for 6 hours in an oxygenbackground pressure of 1 atmosphere. In this case, the microstructureturns out as a fully crystallized randomly oriented BST phase. However,both phases as shown in FIGS. 2a and 2 b and FIGS. 3a and 3 b haverelatively large spaces (i.e., pores) between the grains. The randomlyoriented grains in the BST buffer layer could serve as seeds for thesubsequently deposited crystalline BST film. The random orientation ofthe grains and the spaces (i.e., pores) between the grains in the bufferlayer could help not only relieving film strain but also growingcrystalline grains in the subsequently deposited dielectric film.

[0031] Next, as a part of the tunable dielectric film, a BST (x=0.4)thin film (0.3 μm thick) was deposited on top of the BST buffer layer.All deposition conditions including laser parameters were the same as inthe BST buffer layer deposition step except a substrate temperature of750° C. and a laser repetition rate of 5 Hz were used. FIG. 2c shows aSEM image of BST (x=0.4) film as deposited at 750° C. on BST bufferlayer of FIG. 2b, indicating that the pores in the BST buffer layer arefilled with an aggregation of small grains (<0.1 μm) of subsequentlydeposited BST film.

[0032] Next, the BST film deposited on the BST buffer layer was annealedin flowing oxygen at 1,000° C. for 6 hours. FIG. 2d and FIG. 3c show aSEM image and a XRD pattern, respectively, of the BST film annealed at1,000° C. in flowing oxygen for 6 hours. The grain size is observed togrow significantly up to ˜0.5 μm by reducing grain boundaries whichexisted in the as-deposited BST film during the post annealing process,as shown in FIGS. 2c and 2 d. An analysis of the XRD pattern of FIG. 3cindicates that the annealed BST film deposited with the BST buffer layeris a single perovskite phase but not a single crystallographicorientation, which was the same as in an XRD pattern (not shown) foras-deposited BST film with BST buffer layer. Also, the ratio of peakintensities is very close to the corresponding powder diffractionpattern as shown in FIG. 3d, indicating that the BST film with BSTbuffer layer is a randomly oriented polycrystalline phase.

[0033] The lattice structure of the BST film with BST buffer layer wasdetermined as a cubic phase (a₀=3.966 Å) based on XRD patterns of 11reflection peaks (i.e., (100), (110), (111), (200), (210),(211),(220),(310),(311), (222), and (321) peaks) from the BST film. Itis also noted that the lattice parameter (3.966 Å) of the BST (x=0.4)film with BST (x=0.4) buffer layer is very close to the equilibriumlattice parameter (3.965 Å) of bulk BST (x=0.4). These structural (i.e.,microstructure and lattice structure) analyses indicate that film straindue to the film—substrate mismatches (i.e., lattice and thermalexpansion) could be effectively avoided using the buffer layer duringfilm deposition and post annealing processes. Advantageous annealingeffects (i.e., reducing defects and growing grains) of the film materialwere achieved without thermally induced film strain due tofilm—substrate thermal expansion mismatch during post-deposition heattreatment.

[0034] In addition to the BST film deposition with the BST buffer layeraccording to the present invention, a BST (x=0.4) thin film (0.3 μmthick) was grown directly on a (001) MgO substrate without a BST (x=0.4)buffer layer at 750° C. in an oxygen background pressure of 200 mTorr bypulsed laser deposition. This is a typical BST film deposition methodusing pulsed laser deposition (PLD). This additional experimentdemonstrates how film strain caused by the film—substrate mismatches(i.e., lattice and thermal expansion) without a BST buffer layer affectsthe microstructure, lattice structure, and microwave dielectricproperties of the film.

[0035] A BST (x=0.4) thin film deposited directly on a (001) MgOsubstrate without a BST (x=0.4) buffer layer was found by XRD to besingle phase and exclusively oriented in the (001) direction (i.e.,epitaxial film). FIGS. 4a and 4 b show x-ray diffraction (XRD) patternsof symmetric (004) peaks and asymmetric (024) peaks, respectively, ofthe BST film annealed in flowing O₂ at 1,000° C. for 6 hours.

[0036] To determine the film strain (i.e., lattice structure), thelattice parameters along the surface normal (a_(normal)) and in theplane of the film (a_(in-plane)) were determined from XRD patterns of(004) and (024) reflections for (001) oriented epitaxial BST filmdeposited directly on (001) MgO substrate without a BST buffer layer asshown in FIGS. 4a and 4 b, respectively. Diffraction from the MgOsubstrate was used as an internal standard to reduce errors associatedwith measurement. Each BST diffraction peak was fitted with two Gaussianfunctions by considering four factors of peak shape (i.e., ratios ofheight, width, and area of K_(α1) and K_(α2) peaks, and distance betweenK_(α1) and K_(α2) peaks) after removing the background. The uncertaintyof the lattice parameter is typically less than 0.001 Å. The results of2Θ measurement and lattice parameter calculation in FIGS. 4a and 4 b arepresented in Table 1. Table 1 lists 2Θ peak values of FIGS. 4a and 4 band the calculated lattice parameters (i.e., surface normal andin-plane) of epitaxial BST film deposited on (001) MgO without BSTbuffer layer. TABLE 1 MgO substrate BST (x = 0.4) film (hkl) (004) (024)(004) (024) 2Θ [degree] 93.9743 109.7669 102.0780 120.7885 a_(normal)[Å] 4.2112 3.969 a_(in-plane) [Å] 4.2112 3.969

[0037] The in-plane lattice parameter of epitaxial BST (x=0.4) filmdeposited directly on (001) MgO substrate without the BST (x=0.4) bufferlayer is observed to be 0.2% larger than the normal lattice parameter,resulting in an in-plane tetragonal lattice distortion(a_(in-plane)>a_(normal)), although the structure for the correspondingbulk BST is a cubic structure.

EXAMPLE 2

[0038] Characterization of small signal microwave properties of a planarvaractor containing a strain-relieved and defect-reduced BST (x=0.4)thin film with a BST (x=0.4) buffer layer according to the presentinvention was made at room temperature using a gap-coupledhalf-wavelength (λ/2) stripline resonator connected to an HP 8510Cnetwork analyzer.

[0039]FIG. 5a illustrates a planar varactor 18 based on a strained butdefect-reduced epitaxial BST (x=0.4) film 20 deposited directly on a(001) MgO substrate 10 without a BST (x=0.4) buffer layer, and thenannealed in flowing oxygen at 1,000° C. for 6 hours. FIG. 5b illustratesa planar varactor 24 based on a strain-relieved and defect-reducedrandomly oriented crystalline BST (x=0.4) film 16 deposited on a (001)MgO substrate 10 with a BST (x=0.4) buffer layer 12 in accordance withthe present invention, and then annealed in flowing oxygen at 1,000° C.for 6 hours. For both types of varactor shown in FIGS. 5a and 5 b,single-gap planar electrodes 22 with gaps G of from 6 to 18 μm aredeposited on top of the BST (x=0.4) films through a photolithographylift-off technique by e-beam evaporation of 0.5 μm thick Au with a thin(300 Å thick) adhesion layer of Ti.

[0040] The determination of capacitance (C_(p)) and dielectric qualityfactor (Q=1/tan δ) of the planar varactor as a function of DC biasvoltage was made at about 2 GHz and 8 GHz by including the varactor intoa break of the resonator stripline and by measuring loaded Q (Q_(L)),insertion loss (S₂₁), 3 dB width (Δf) of resonant frequency, andresonant frequency (f_(r)). It is noted that dielectric Q measurementsusing a stripline resonator should be limited by unloaded Q (Q₀) of theresonator with varactor practically without dielectric loss (i.e.,Q₀=300 and 500 for 2 GHz and 8 GHz resonators in this experiment,respectively). Dielectric tuning of capacitance (% T(C_(p))) can bedefined as {[C_(p)(0 V_(DC))−C_(p)(V_(DC))]/C_(p)(0 V_(DC))}×100.Dielectric constant of BST (x=0.4) film (ε_(film)) was extracted fromcapacitance (C_(p)) value and varactor dimension by conformal mappingtransformation.

[0041]FIG. 6a is resonance curve of a stripline resonator incorporatinga planar varactor as illustrated in FIG. 5a based on a strained butdefect-reduced epitaxial BST (x=0.4) film deposited directly on (001)MgO without a BST (x=0.4) buffer layer and then annealed in flowing O₂at 1,000° C. for 6 hours. FIG. 6b is a resonance curve of a striplineresonator incorporating a planar varactor as illustrated in FIG. 5bbased on a strain-relieved and defect-reduced randomly orientedcrystalline BST (x=0.4) film deposited on (001) MgO with a BST (x=0.4)buffer layer, and then annealed in flowing O₂ at 1,000° C. for 6 hoursaccording to the present invention. The resonance curves were measuredat about 8 GHz and room temperature as a function of DC bias voltage tothe varactor. The 8 GHz dielectric properties (i.e., dielectric Q anddielectric tuning) of the films were extracted from the resonanceresponses of FIGS. 6a and 6 b, respectively, and are presented in Table2. Table 2 is a table of 8 GHz dielectric properties of planar varactorsof FIGS. 5a and 5 b. TABLE 2 BST V_(DC) Gap Q Q % Varactor Cp ε_(film)(V) (μm) (0 V_(DC)) (200 V_(DC)) T (Cp) 0.443 526 0-200 18 21 142 280.435 371 0-200 18 146 152 16

[0042] Table 2 shows a significant difference in dielectric Q values interms of magnitude and DC bias voltage dependency between the BST filmwithout the BST buffer layer of FIG. 5a and the BST film with the BSTbuffer layer of FIG. 5b. The BST film with the BST buffer layeraccording the present invention shows more stable DC bias voltagedependency over the bias range (i.e., 0 to 200 V_(DC)) and higherdielectric Q at 0 V_(DC) than the typical prior art BST film.

[0043] More characterizations of small signal microwave properties ofplanar varactor containing strain-relieved and defect-reduced BST(x=0.4) thin film with BST (x=0.4) buffer layer according to the presentinvention were made as a function of varactor gap size ranging from 6 to18 μm. FIGS. 7a and 7 b are plots of dielectric tuning and dielectric Qof varactor as a function of varactor gap size at 2 GHz and at DC biasvoltages up to 300 V except the circled data points with V_(MAX)=50-200V_(DC) as noted close to the circles for planar varactors based on BSTfilm with BST buffer layer according to the present invention. Severaldifferent varactors with a same gap size are characterized for certaingap sizes, and the open-box data points in FIG. 7a indicates dielectricQ of varactor at 0 V_(DC).

[0044]FIGS. 8a and 8 b are plots of dielectric tuning and dielectric Qof varactors as a function of varactor gap size at 8 GHz and at DC biasvoltages up to 300 V, except the circled data points withV_(MAX)=150-200 V_(DC) as noted close to the circles for planarvaractors based on the BST film with BST buffer layer according to thepresent invention. Several different varactors with a same gap size arecharacterized for different gap sizes, and the open-box data points inFIG. 8b indicate dielectric Q of varactor at 0 V_(DC). At bothfrequencies (2 GHz and 8 GHz), the dielectric tuning tends to increaseas the gap size decreases as shown in FIG. 7a and FIG. 8a. However, incase of the dielectric Q, no clear trend with the gap sizes is observed.High dielectric Q's are achieved independently of the gap sizes when thegaps ranged from 6 to 18 μm.

[0045] Whereas particular embodiments of this invention have beendescribed above for purposes of illustration, it will be evident tothose skilled in the art that numerous variations of the details of thepresent invention may be made without departing from the invention asdefined in the appended claims.

What is claimed is:
 1. A method of making a tunable dielectric thinfilm, the method comprising the steps of: depositing a buffer layer on alow dielectric loss substrate; and depositing a layer of crystallinedielectric film on the buffer layer, wherein the buffer layer and thecrystalline dielectric film comprise barium strontium titanate andwherein the barium strontium titanate eventually comprises a randomlyoriented crystalline cubic phase in each of the buffer layer and thecrystalline dielectric film.
 2. The method of claim 1, wherein in thedepositing steps comprise at least one of RF sputtering, pulsed laserdeposition, and metal-organic chemical vapor deposition.
 3. The methodof claim 1, wherein the barium strontium titanate is of the formulaBa_(1-x)Sr_(x)TiO₃, where x is from about 0.0 to about 0.6.
 4. Themethod of claim 1, wherein the buffer layer is deposited at a firsttemperature, and the layer of crystalline dielectric film is depositedat a second temperature that is higher than the first temperature. 5.The method of claim 1, wherein the randomly oriented crystalline cubicphase in the buffer layer is prepared by depositing at room temperatureand then heating at a temperature higher than about 750° C. for morethan about 20 minutes.
 6. The method of claim 1, further comprising thestep of annealing the buffer layer after it was deposited on the lowdielectric loss substrate.
 7. The method of claim 1, further comprisingthe step of annealing the crystalline dielectric film after it wasdeposited on the dielectric buffer layer.
 8. The method of claim 7,wherein the crystalline dielectric thin film is annealed at atemperature higher than a temperature at which the crystallinedielectric film is deposited.
 9. The method of claim 7, wherein theannealed crystalline dielectric film has an average grain size of fromabout 0.1 to about 0.5 micron.
 10. The method of claim 7, wherein theannealed crystalline dielectric film has a dielectric quality factor Qat 0 V_(CD) of greater than about 100 at a frequency of 2 GHz or 8 GHz.11. The method of claim 7, wherein the annealed crystalline dielectricfilm has a dielectric tuning of at least about 20 percent at 50V/micronat frequencies of 2 GHz and 8 GHz.
 12. The method of claim 7, whereinthe annealed crystalline dielectric film has a dielectric quality factorQ that is substantially equal or increases with DC bias voltages. 13.The method of claim 7, wherein the annealed crystalline dielectric filmsubstantially maintains dielectric tuning at frequencies of 2 GHz and 8GHz.
 14. The method of claim 1, wherein the tunable dielectric thin filmis used within a tunable microwave device.
 15. A method of making atunable dielectric thin film, the method comprising the steps of:depositing a buffer layer on a low dielectric loss substrate; annealingthe buffer layer after it was deposited on the low dielectric losssubstrate; depositing a layer of crystalline dielectric film on thebuffer layer; and annealing the crystalline dielectric film after it wasdeposited on the dielectric buffer layer, wherein the buffer layer andthe crystalline dielectric film comprise barium strontium titanate andwherein the barium strontium titanate comprises a randomly orientedcrystalline cubic phase in each of the annealed buffer layer and theannealed crystalline dielectric film.
 16. The method of claim 15,wherein in the depositing steps comprise at least one of RF sputtering,pulsed laser deposition, and metal-organic chemical vapor deposition.17. The method of claim 15, wherein the barium strontium titanate is ofthe formula Ba_(1-x)Sr_(x)TiO₃, where x is from about 0.0 to about 0.6.18. The method of claim 15, wherein the buffer layer has a thickness offrom about 0.0005 to about 0.2 micron.
 19. The method of claim 15,wherein the buffer layer has a thickness of from about 0.01 to about 0.1micron.
 20. The method of claim 15, wherein the buffer layer is porous.21. The method of claim 15, wherein the crystalline dielectric film hasa thickness of from about 0.1 to about 5 micron.
 22. The method of claim15, wherein the crystalline dielectric film has a thickness of fromabout 0.3 to about 1 micron.
 23. The method of claim 15, wherein thebarium strontium titanate of the buffer layer is deposited at a lowertemperature than the barium strontium titanate of the crystallinedielectric film.
 24. The method of claim 15, wherein the crystallinedielectric film is strain relieved.
 25. The method of claim 15, whereinthe crystalline dielectric film is substantially strain free.
 26. Themethod of claim 15, wherein the low dielectric loss substrate comprisesat least one crystalline material selected from magnesium oxide,aluminum oxide and lanthanum aluminum oxide.
 27. The method of claim 15,wherein the low dielectric loss substrate has a thickness of from about250 to about 500 micron.
 28. The method of claim 15, wherein thedielectric thin film has a dielectric quality factor Q at 0 V_(DC) ofgreater than about 100 at a frequency of 2 GHz or 8 GHz.
 29. The methodof claim 15, wherein the dielectric thin film has a dielectric tuning ofat least about 20 percent at 50 V/micron at a frequency of 2 GHz or 8GHz.
 30. The method of claim 15, wherein the tunable dielectric thinfilm is used within a tunable microwave device.
 31. The method of claim15, wherein the buffer layer is deposited on the low dielectric losssubstrate in an oxygen background pressure of about 200 mTorr.
 32. Themethod of claim 15, wherein the crystalline dielectric film is depositedon the buffer layer and then annealed in flowing oxygen at about 1000°C. for about 6 hours.
 33. A method of making a tunable dielectric thinfilm, the method comprising the steps of: depositing a buffer layer on alow dielectric loss substrate; annealing the buffer layer after it wasdeposited on the low dielectric loss substrate; depositing a layer ofcrystalline dielectric film on the buffer layer; and annealing thecrystalline dielectric film after it was deposited on the dielectricbuffer layer, wherein the annealed buffer layer and the annealedcrystalline dielectric film each have a randomly oriented crystallinecubic phase.
 34. The method of claim 33, wherein in the depositing stepscomprise at least one of RF sputtering, pulsed laser deposition, andmetal-organic chemical vapor deposition.
 35. The method of claim 33,wherein the buffer layer comprises at least one material selected frombarium titanate, strontium titanate, barium calcium titanate, bariumcalcium zirconium titanate, lead titanate, lead zirconium titanate, leadlanthanum zirconium titanate, lead niobate, lead tantalate, potassiumstrontium niobate, sodium barium niobate/potassium phosphate, potassiumniobate, lithium niobate, lithium tantalate, lanthanum tantalate, bariumcalcium zirconium titanate or sodium nitrate, Ba_(1-x)Sr_(x)TiO₃(0≦x≦1), Ba_(x)Ca_(1-x)TiO₃ (0.2≦x≦0.8), Pb_(x)Zr_(1-x)SrTiO₃(0.05≦x≦0.4), KTa_(x)Nb_(1-x)O₃ (0≦x≦1), Pb_(x)Zr_(1-x)TiO₃ (0≦x≦1), andmixtures and composites thereof.
 36. The method of claim 33, wherein thecrystalline dielectric film comprises at least one material selectedfrom barium titanate, strontium titanate, barium calcium titanate,barium calcium zirconium titanate, lead titanate, lead zirconiumtitanate, lead lanthanum zirconium titanate, lead niobate, leadtantalate, potassium strontium niobate, sodium barium niobate/potassiumphosphate, potassium niobate, lithium niobate, lithium tantalate,lanthanum tantalate, barium calcium zirconium titanate or sodiumnitrate, Ba_(l-x)Sr_(x)TiO₃ (0≦x≦1), Ba_(x)Ca_(1-x)TiO₃ (0.2≦x≦0.8),Pb_(x)Zr_(1-x)SrTiO₃ (0.05≦x≦0.4), KTa_(x)Nb_(1-x)O₃ (0≦x≦1),Pb_(x)Zr_(1-x)TiO₃ (0≦x≦1), and mixtures and composites thereof.
 37. Themethod of claim 33, wherein the low dielectric loss substrate comprisesat least one crystalline material selected from magnesium oxide,aluminum oxide and lanthanum aluminum oxide and/or with additionaldoping elements selected from manganese, iron, tungsten, alkali earthmetal oxides, transition metal oxides, rare earth metal oxides,silicates, niobates, tantalates, aluminates, zirconates or titanates.38. The method of claim 33, wherein the crystalline dielectric thin filmis annealed at a temperature higher than a temperature at which thecrystalline dielectric film is deposited.
 39. The method of claim 33,wherein the annealed crystalline dielectric film has a dielectricquality factor Q at 0 V_(DC) of greater than about 100 at a frequency of2 GHz or 8 GHz.
 40. The method of claim 33, wherein the annealedcrystalline dielectric film has a dielectric tuning of at least about 20percent at 50V/micron at frequencies of 2 GHz and 8 GHz.
 41. The methodof claim 33, wherein the annealed crystalline dielectric film has adielectric quality factor Q that is substantially equal or increaseswith DC bias voltages.
 42. The method of claim 33, wherein the annealedcrystalline dielectric film substantially maintains dielectric tuning atfrequencies of 2 GHz and 8 GHz.
 43. The method of claim 33, wherein thetunable dielectric thin film is used within a tunable microwave device.